The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium.
The FCC process is carried out by contacting the starting material--whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons--with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat; a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC unit cracks gas oil or heavier feeds into a broad range of products. Cracked vapors from the FCC reactor enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, cycle oil and heavy residual components. The gasoline cut includes both light and heavy gasoline components. A major component of the heavy gasoline fraction comprises heavy single ring aromatics.
It has long been desired to process more than one feedstock in an FCC unit. FCC processes have been proposed for cracking multiple feeds in a single riser. U.S. Pat. No. 4,392,643 specifically discloses the cracking of first a gas oil mixture followed by cracking of a naphtha boiling range stream in a single FCC riser. It is also known from U.S. Pat. No. 5,389,232 to use a heavy naphtha boiling range hydrocarbon as a quench in an FCC riser to control the riser temperature and the cracking of a gas oil feed.
Recent advances in FCC process arrangements have led to significant reductions in the amount of the coke laid down on the catalyst in the reaction zone. Improvements to the distribution of feed and the separation of products from catalyst have largely contributed to the reduction in coke production. While reduction in coke is desirable overall, it has the effect of limiting the operating temperature of the regeneration zone and the resulting temperature of the regenerated catalyst. Lower regenerated catalyst temperatures reduce the reaction temperature in the reactor riser. Lower reaction temperatures shift the cracking reaction away from thermal cracking and toward catalytic cracking. To maintain conversion it is often necessary to circulate more catalyst with the feed. Circulating more catalyst can be an imperfect solution to reduced conversion. First, the higher catalyst circulation rate may tend to further reduce coke lay down resulting in a downward temperature spiral for the regenerated catalyst as the temperature of the catalyst decreases with increased circulation and the circulation must continue to increase with decreasing catalyst temperature. In addition, in many existing units the catalyst circulation rate may be limited so that increasing the catalyst to feed ratio may come at the expense of limiting feed throughput.
Relatively lower regenerated catalyst temperature poses special problems for conversion zone arrangements. Circulation of the catalyst through an additional conversion zone will have an inherent cooling effect. Moreover, in the vast majority of cases the additional conversion zone will effect an endothermic reaction. Therefore, the additional conversion zone operates as a catalyst cooler that further removes heat from the process and continues the depression of regenerated catalyst temperatures. The problem becomes further exacerbated where the additional conversion would benefit from higher operating temperatures, such as in the case of thermal cracking, but high temperature catalyst is unavailable.
The available methods of increasing regenerated catalyst temperature are not commercially attractive. Reducing the hydrocarbon conversion and/or the recovery of hydrocarbons from the process will increase regenerator temperature, but at the expense of overall process efficiency. Various promoters and combustion material may be added to the regenerator to promote CO combustion or to combust additional fuel. Both of these alternatives add expense and complexity to the operation of the regenerator.